Magnetic devices present many problems to the designer. These problems include space allocations, especially mounted height dimensions, and material costs and its electrical performance. To achieve a desired result in one of these areas usually involves an undesirable compromise to a less desirable result in another area. Two of the most pressing problems at present concern designing magnetic devices that have physical dimensions suitable for modular circuit packs and magnetic devices having acceptable electrical performance while still permitting low cost manufacture.
The present practice of embodying components within circuit modules in a common housing structure requires circuits having low profile dimensions to facilitate their packaging in close proximity to one another in the housing structure where they may all interconnect into a common backplane system. This arrangement has limited the physical size of the magnetic components that may be included in each individual circuit module. These size limitations are particularly critical and difficult to meet in power supply circuit modules which process substantial amounts of power.
Some prior arrangements to adapt magnetic structures to meet low profile requirements have included using air core magnetic devices with the conductor printed in a spiral format on the circuit board of the module. In some arrangements a magnetic material has been additionally deposited on the circuit board in close proximity with the printed conductor windings. In terms of operational efficiency, flux containment and control of parasitics, these arrangements have had limited success. However, the advantage of low profile and ease of manufacture has in many instances dictated their use. One particular example of a different type of a low profile magnetic structure is disclosed in U.S. Pat. No. 4,134,091 issued to N. A. Rogers on Jan. 9, 1979. The structure disclosed therein consists of a plurality of toroidal cores which are strung in bead-like fashion on the transformer windings. The transformer windings are preferably coaxial with one of the windings comprising electrically conductive tubing to support or contain the other windings. While a low profile is achieved, the transformer assembly is loosely structured and very expensive to manufacture. Hence, a magnetic structure retaining the advantage of easily automated manufacture and low profile combined with the further advantage of well defined flux paths, low parasitics and efficiency is very desirable.